264 HISTORICAL PALAZONTOLOGY. 
Globigerina, Rotalia (fig. 187), and Zextularia—groups which 
are likewise characteristic of the “ooze” of the Atlantic and 
Fig. 187.—Rotalia Boucana. 
Pacific Oceans at great depths. The flints of the Chalk also 
commonly contain the shells of Foraminifera. The Upper 
Greensand has yielded in considerable numbers the huge 
foraminifera described by Dr Carpenter under the name of 
farkerta, the spherical shells of which are composed of sand- 
grains agglutinated together, and sometimes attain a diameter 
of two and a quarter inches. The Cretaceous Sfonges are 
extremely numerous, and occur under a great number of varie- 
ties of shape and structure; but the two most characteristic 
genera are S7phonia and Ventriculites, both of which are ex- 
clusively confined to strata of this age. The Siphonie (fig. 
188) consist of a pear-shaped, sometimes lobed head, supported 
by a longer or shorter stem, which breaks up at its base into a 
number of root-like processes of attachment. The water gained 
access to the interior of the Sponge by a number of minute 
openings covering the surface, and ultimately escaped by a 
single, large, chimney-shaped aperture at the summit. In some 
respects these sponges present a singular resemblance to the 
beautiful ‘ Vitreous Sponges” (/Zo/tenia or Pheronema) of the 
deep Atlantic ; and, like these, they were probably denizens 
of a deep sea. The Ventriculites of the Chalk (fig. 189) is, 
however, a genus still more closely allied to the wonderful 
flinty Sponges, which have been shown, by the researches of 
the Porcupine, Lightning, and Challenger expeditions, to live 
half buried in the calcareous ooze of the abysses of our great 
oceans. Many forms of this genus are known, having ‘ usu- 
ally the form of graceful vases, tubes, or funnels, variously 
ridged or grooved, or otherwise ornamented on the surface, 
frequently expanded above into a cup-like lip, and continued 
below into a bundle of fibrous roots. The minute structure of 
these bodies shows an extremely delicate tracery of fine tubes, 
sometimes empty, sometimes filled with loose calcareous mat- 
